The present invention relates to a discharge-excited laser apparatus, and more particularly to a switch for the discharge-excited gas laser apparatus.
FIG. 1 illustrates a conventional high-voltage switch using a thyratron for a discharge-excited gas laser apparatus, which is for example, shown in a EMG50E product catalogue issued by Lambda Physics Inc. In this Figure, reference number 7 indicates a thyratron, which is a high-voltage switch. Reference number 4 indicate capacitors charged up to a high voltage, which are arranged side by side in a large number in parallel connection. The terminal of the thyratron 7 at a high-voltage switch side and the capacitors 4 are connected with a conductive plate 10. A conductive container 8 for accommodating the thyratron 7 therein is connected with the low-voltage terminal of the thyratron 7. A lid 9 of the thyratron container 8 is made of an insulating material in order to maintain insulation between the terminals of the thyratron 7. A lower conductive plate 6 is connected to the capacitors 4 in parallel. Peaking capacitors 15 of a capacity shifting circuit are connected in parallel in a large number to both sides of the lower conductive plate 6. Reference number 17 indicates a conductive plate, which forms a capacity shifting loop with 4-10-7-8-17-15 -6-4. A pair of mutually opposing discharge electrodes 16, which extend in the direction of an optical axis, are respectively installed on the lower conductive plate 6 and the conductive plate 17. The high-voltage switch shown in FIG. 1 is expressed as shown in FIG. 2 in terms of an electrical circuit diagram. This circuit forms the capacity shifting circuit. However, FIG. 1 does not show reactors 13 and 14 and a high-voltage power source 11.
Next, the operations of the high-voltage switch will be described. The switch shown in FIG. 1 is equivalent to the electrical circuit diagram shown in FIG. 2. In this case, the high-voltage switch 12 corresponds to the thyratron 7. The operations in FIG. 2 are such that electric charge is accumulated in the charging capacitor 4 from a high-voltage power source 11 via the charging reactors 13 and 14, and, when the switch 12 is closed thereafter, the electric charge in the capacitor 4 shifts to the peaking capacitor 15. When electric charge is built up in the peaking capacitor 15, the electric charge is promptly fed to the laser discharge electrodes 16. Thus, in FIG. 1 in which the thyratron 7 corresponds to the high-voltage switch 12, the electric charge charged in the charging capacitor 4 is shifted to the peaking capacitor 15 via the loop formed of 4-10-7-8-17-15-6, when ignition occurs in the thyratron. As a result, the voltage between the electrodes 16 rises sharply, and, as the discharge space between the electrodes breaks down, the energy in the peaking capacitors 15 is put into the discharge field. Thus, the gas in the discharge space is excited, and the stimulated emission produces a laser beam.
The above described conventional high-voltage switch for a discharge-excited laser has had the problems that it fails to transmit the electric charge in any uniform manner, since the electric charge will be in a coaxial form in the thyratron part because of the shape of the thyratron, even if the charging capacitors are arranged in parallel in the longitudinal direction of the electrodes to put the electric charge uniformly in the longitudinal direction to the electrodes, so that the high-voltage switch fails to transmit the electric charge accumulated in the charging capacitors as it is in its form of arrangement to the peaking capacitor, and eventually fails to charge the peaking capacitor uniformly and fails to feed any electric charge uniformly in the longitudinal direction of the electrodes into the discharge field. Therefore, the voltage in the peaking capacitors lacks for uniformity in the longitudinal direction of the electrodes and also the discharge in the longitudinal direction of the electrodes shows a lack of uniformity, so that the laser generating efficiency declines.
By the way, the rise-up velocity dv/dt of voltage between the discharge electrodes will be higher as the shifting velocity in the above-mentioned shifting loop increases. It is known that more stable discharge will be obtained in, for example, an excimer laser, when the above-mentioned dv/dt value increases. Therefore, it is a usual practice to arrange the charging capacitors 4 and the peaking capacitors 15 in parallel with the direction of the optical axis in relation to the discharge electrodes 16, thereby forming a construction which can achieve a reduction of a stray inductance to the maximum extent possible. However, if a thyratron is used as a high-voltage switch as in the conventional switch, the electric current converges in the thyraton part in the shifting loop, and, consequently, the overall inductance of the shifting loop cannot be reduced to any level lower than 200 nH (of which 100 to 150 nH is in the thyratron part).
Moreover, as a thyratron is sensitive to changes in temperature, it also has had such short-comings as the requirement of a preheating time at the start-up of an apparatus and the necessity of strict temperature control.